Field of the Invention
The invention relates to a microporous membrane of polyethylene-based composite material for lithium ion batteries and a preparation method thereof.
Description of the Related Art
Typical commercial polyolefin microporous membranes include: a three-layer of PP/PE/PP composite membrane prepared by a dry method, a single layer of PE membrane having a large molecule weight prepared by a wet method, and a microporous physical gel membrane prepared by a solvent-induced phase separation method. A typical microporous physical gel membrane is a polyvinylidenefluoride-hexafluoro propylene (PVDF-HFP) copolymer microporous physical gel membrane prepared by Bellcore process. The micropores of the PVDF-HFP gel membrane have slightly large pore size, approximately between 0.5 and 2 μm; and the membrane is not strengthened by hot stretching, thus, the mechanical strength of the membrane is low and is not applicable to process requirements like winding.
The three-layer of PP/PE/PP composite membrane prepared by the wet method has the following shortages:
1. The strength and toughness of the membrane are not good. The membrane is prone to be torn in a transverse direction, and the transverse elongation at break is smaller than 20%.
2. Although the middle microporous layer employs PE that has the shutdown ability at a high temperature of between 135 and 145° C., the fusion point thereof is limited and drawbacks of large thermal shrinkage and insufficient high temperature rupture resistance still exist in the hot stretched PP microporous layers at the temperature of 130° C.
3. Compared with the PVDF-HFP physical gel membrane or the coated membrane, the membrane prepared by the dry method has low bonding capability with the electrode pieces during the hot press.
The existing PE membrane prepared by the wet method has the following disadvantages:
1. During the preparation, the polyethylene powder having a large molecular weight and a high density is mixed with the high temperature compatilizer, i. e., the liquid paraffin oil, heated and blended. However, the fluctuation exists in the solid content of the slurry, the stability of the feeding and the consistency in the melt content are not good. The stability and the consistency of the product are affected.
2. Because the hot stretching strengthening process is employed, the thermal shrinkage is slightly large at the high temperature of 120° C. above, and the safety of the battery cannot be ensured.
3. Compared with the PVDF-HFP physical gel membrane or the coated membrane, the PE membrane prepared by the wet method is also lack of the bonding capability with the electrode pieces, the elasticity in the thickness direction, or the stress absorption capability. The PE membrane prepared by the wet method cannot meet the demands of the power battery on aspects of the safety and the cycle life.